Chirpy types heading off youthful disease (7)
I believe the answer is:
rickets
'disease' is the definition.
(rickets is a kind of health problem)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for rickets that I've seen before include "Childhood disease caused by a deficiency of vitamin D" , "Bone disease, caused by vitamin D deficiency" , "Children's disease" , "Bone-distorting disease" .)